There are physical differences, QLC requires more precise hardware, since you need to distinguish between more charge levels. But you can display a low-quality picture on a high-definition screen, or in a camera sensor average 4 physical pixels to get a virtual one, same thing here, you combine together some charge levels for increased reliability.
Put another way, you can turn a QLC into a TLC, but not the other way around.
The memory cells are identical. The peripheral circuitry for accessing the memory array gets more complicated as you support more bits per cell, and the SRAM page buffers have to get bigger to hold the extra bits. But everyone designs their NAND chips to support operating with fewer bits per cell.
Sometimes TLC and QLC chips will be made in different sizes, so that each has the requisite number of memory cells to provide a capacity that's a power of two. But it's just as common for some of the chips to have an odd size, eg. Micron's first 3D NAND was sold as 256Gbit MLC or 384Gbit TLC (literally the same die), and more recently we've seen 1Tbit TLC and 1.33Tbit QLC parts from the same generation.
